In 1841, Professor Forbes, along with M. Agassiz, and others, made a successful ascent of the great Swiss mountain, the Jungfrau, whose summit is 13,720 feet above the level of the sea.

Of six travellers and seven guides who formed the party, four of each reached the top—viz., of the former, MM. Forbes, Agassiz, Desor, and Duchatelies; of the latter, Jacob Leutvold (who ascended the Finster Aarhorn,) Johan Jannon, Melchior, Baucholzer, and Andreas Aplanalp. They left the Grimsel on the morning of the 27th of August, 1841, ascended the whole height of the Ober-Aar Glacier, and descended the greater part of that of Viesch. Crossing a col to the right, they slept at the chalet of Aletsch, near the lake of that name. This was twelve hours' hard walking, the descent of the glaciers being difficult and fatiguing. Next day, the party started at six a.m., having been unable sooner to procure a ladder, to cross the crevices; they then traversed the upper part of the glacier of Aletsch in its whole extent for four hours and a half, until the ascent of the Jungfrau began.

The party crossed with great caution extensive and steep fields of fresh snow, concealing crevices, till they came to one which opened vertically, and behind which rose an excessively steep wall of hardened snow. Having crossed the crevices with the ladder, they ascended the snow without much danger, owing to its consistency. After some similar walking they gained the col, which separates the Aletsch Glacier from the Rothal, on the side of Lauterbrunnen, by which the ascent has usually been attempted. Thus, the travellers, although now at a height of between 12,000 and 13,000 feet, had by far the hardest and most perilous part of the ascent to accomplish. The whole upper part of the mountain presented a steep, inclined surface of what at first seemed snow, but which soon appeared to be hard ice. This slope was not less than 800 or 900 feet in perpendicular height, and its surface (which Professor Forbes measured several times with a clinometer,) in many places rose at 45 degrees, and in few much less; and all Alpine travellers know well what an inclined surface of 45 degrees is to walk up. Of course, every step taken was cut with the hatchet, whilst the slope terminated below, on both sides in precipices some thousand feet high. After very severe exertion, they reached the top of this great mountain, at four p.m. The summit was so small that but one person could stand upon it at once, and that not until the snow had been flattened. The party returned as they came up, step by step, and backwards, and arrived at the chalets of Aletsch, and by beautiful moonlight, at half-past eleven at night.